Rigging an extendable tentacle
You can do this using Spline Ik and Bendy bones. Here is the result 

1. Create the bones for your tentacle.



Under the armature settings in display, select B-Bone

Enter Edit Mode, select each bone and change the segments for it. Note more segments equals a smoother transition.

Create a Bezier Curve, select the bent handle and scale it to 0 in the y-axis to straighten it. Subdivide it to add as many control points as you want
https://imgur.com/a/oGKtL3u
Align the Curve with the bones.
https://imgur.com/a/JCGf296
Enter Pose Mode, Select the last bone, and add a Spline IK constraint to it. Select the curve and change the chain length to the number of segments the Bendy Bone has. 
Parent the Bones to the mesh. You can now hide the bones.
8.Select each part of the spline and press ctrl + h and choose Hook to new object
Note: I added a subdivision surface modifier on the mesh above the armature to get clean deformation.